The present invention relates to a new and improved method of regulating a power plant containing a gas turbine assembly operated in an open cycle, the gas turbine assembly essentially comprising a compressor for the combustion air, a combustion chamber and a gas turbine, wherein a heat exchanger is arranged at the exhaust side of the gas turbine where there is utilized the thermal energy still contained in the exhaust gases of the gas turbine. The invention also relates to apparatus for the performance of the aforesaid method.
The temperature of the combustion gases at the inlet to a gas turbine is limited because of the material from which the gas turbine is constructed. This can be accomplished by the infeed of a considerable amount of excess air. In order not to exceed the dew point of the exhaust gases, a minimum exhaust gas temperature must be maintained in the flue, which, depending upon the sulphur content of the fuel, is approximately between 100.degree. C. and 180.degree. C. This leads to considerable thermal losses. At the maximum output of the gas turbine its thermal efficiency amounts to about 30% and the total utilization of the fuel, i.e., the generated output of the gas turbine plus the employed residual heat amounts to about 68% or 83% for exhaust gas temperatures in the flue of 180.degree. C. and 100.degree. C., respectively.
In order to reduce the generated electrical output for an open type gas turbine, at constant rotational speed (synchronous operation) the temperature of the combustion gas is lowered in front of the turbine by reducing the fuel infeed. This is associated with a reduction in the efficiency. Since, however, the combustion air-compressor delivers a practically constant quantity of air, the excess air of the combustion gases is still further increased in relation to that at full load operatin and, thus, there is further increased the exhaust gas losses per kilogram of fuel. With decrease of the inlet temperature to the gas turbine there, of course, also drops the exhaust temperature, and in conjunction therewith there also is reduced the useful temperature range of the exhaust gases. Hence, with decreasing output of the gas turbine there exists an increasingly poorer utilization of the fuel.
Another possibility is to open an exhaust valve at partial load and to exhaust excess air. However, with this procedure valuable energy is lost, likewise leading to a poorer utilization of the fuel.
It is for these reasons that the gas turbines of heating and power plants, where the waste heat is used for heating purposes, are generally operated at intermittent full load. A continuous operation, where full load and partial load continuously change and as the same is possible in the case of back-pressure-steam turbines or with closed-cycle gas turbines, cannot be recommended for economical and ecological reasons for open-cycle gas turbines.
In the case of power plants utilizing combined gas and steam-turbines, there is counteracted the impairment of the efficiency during partial load by additionally firing the boiler. With practically stoichiometric combustion, the excess air in the exhaust gases of the gas turbine is almost completely utilized. Nonetheless, even with these power plants the efficiency is quite considerably reduced with decreasing partial load.